Process for lowering the magnetic reorientation temperature of samarium orthoferrite



Dec. '1, 1970 E. M. GYORGY ETA!- 3,544,265

PROCESS FOR LOWERING THE MAGNETIC REORIENTATION TEMPERATURE OF SAMARIUM ORTHOFERRITE Filed Nov. 15, ;967

I80- F/G.

5 |60- :5 S? i: I40- 80 I 1 l I I 0 I00 200 300 400 500 HYDROGEN TREATMENT TEMPERATURE DEGREES C FIG. 2

' m 5% |40 GIT- O C ma: 5 I20- 5&- Z 0 2 I00 I l l I l I 0 4 8 l2 I6 20 24 TIME OF HYDROGEN TREATMENT HOURS 20 FIG. 3 22a 23a 24a 22b 23b 24b f2! O O Q 22, T? H 231 I 24 NU%EE""T'ING SENS'NG 48 A SOURCE ME NS E M GVORGY MEMO J P'REMf/KA 4 'ATTO NFV United States Patent 3,544,265 PROCESS FOR LOWERING THE MAGNETIC RE- ORIENTATION TEMPERATURE OF SAMAR- IUM ORTHOFERRITE Ernst M. Gyorgy, Madison, Joseph P. Remeika, Warren Township, Somerset County, N.J., assiguors to Bell Telephone Laboratories, Incorporated, Murray Hill, NJ., a corporation of New York Filed Nov. 15, 1967, Ser. No. 683,289 Int. Cl. C22b 59/00; C01g 49/00, 57/00 US. C]. 23-21 4 Claims ABSTRACT OF THE DISCLOSURE The magnetic reorientation temperature of samarium orthoferrite, SmFeO is reduced to a temperature centering about -85 C. by heating in hydrogen.

BACKGROUND OF THE INVENTION Field of the invention Samarium orthoferrite crystals treated in accordance with the invention are of interest in a large class of devices including those which depend for their operation on the nucleation and propagation of single wall magnetic domains of restricted cross-sectional area.

DESCRIPTION OF THE PRIOR ART The operation of single wall magnetic domain devices is often dependent upon or facilitated by at least temporary, at least local, attainment of a temperature at or near the magnetic reorientation temperature. This temperature, designated by some as a spin flop transition, is that at which the magnetic easy direction switches from one to another crystallographic direction. Device use of the exemplary uniaxial canted spin antiferromagnetic materials often takes the form of a sheet, with the easy direction lying normal to the surface. Operation in certain modes requires local switching of isolated regions 'by 180 and/ or propagation of such nucleated domains within the sheet. Such switching is accomplishable by application of a local field while the temperature of the material is varied so as to carry it through and finally away from the reorientation temperature. Alternatively, such switching is accomplishable by local variation of temperature, so as to restrict the switching accomplished by application of a gross field. It has also been observed that propagation of such nucleated domains is facilitated by operation near such a reorientation temperature.

From a practical standpoint, it is significant that within the promising class of rare earth orthoferrites, only samarium orthofern'te has a reorientation temperature significantly above the boiling point of liquid nitogen. The transition in this material ordinarily occurs at tempera tures of the order of 175 C. or higher. Since convenient operation dictates temperatures near room temperature, it has been considered desirable to tailor materials having reorientation temperatures somewhat lower than 175 C. This has been accomplished by blending samarium orthoferrite with any of the other magnetic rare earth orthoferrites. Such compositional complexity is, however, an obstacle to the achievement of a high degree of compositional homogeneity and crystalline perfection. Both are suggested by the small domain dimensions desired for high information packing density.

SUMMARY OF INVENTION In accordance with the invention, it has been determined that samarium orthoferirte heat treated in the presence of hydrogen manifests lowered reorientation tem- 3,544,265 Patented Dec. 1, 1970 V See BRIEF DESCRIPTION OF THE DRAWING FIG. I, on coordinates of magnetic reorientation temperature and hydrogen treatment temperature, is a plot showing the relationship between these two parameters;

FIG. 2, on coordinates of magnetic reorientation temperature and time of hydrogen treatment in hours for a treatment temperautre of 450 C., is a plot showing the dependence of reorientation on this parameter;

FIG. 3 is a schematic representation of a'device depending for its operation upon the nucleation and propagation of single wall domains in the material of the invention.

DETAILED DESCRIPTION In accordance with the inventive process, crystalline samarium orthoferrite is heated in hydrogen to an elevated temperature, the prefered range of which is usually from 400 C. to 525 C., for a minimum period of about four hours. The preference for this temperature range is based on information such as that plotted on FIG. 1. However, such temperatures merely define the conditions under which a significant reduction in reorientation temperature is attained virtually independent of time of treatment beyond the minimum of about four hours. On the same basis, a more preferred temperature range may be set at from about 400 C. to about 500 C., in turn corresponding with a resulting reorientation temperature of about C. or lower. Neither set of temperature limit is, however, exclusive. Prolonged heating at temperatures somewhat below 400 C. results in significant decrease in the transition temperature. From a commercial standpoint, however, practical extension of the range is of greater interest at the high temperature end. Wellcontrolled procedures may be based on the use of temperatures in excess of 525 C., however, for maximum durations well below .four hours. In work carried out to date, use has not been made of such a higher temperature treatment since reorientation temperature under such conditions is no longer independent of time above a minimum value. In fact, exceeding optimum periods at temperatures above 525 C. is, for example from FIG. I, seen to result in a value for the reorientation temperature appreciably above 100 C.

From data such as that plotted on FIG. 2, it is concluded that reorientation temperature is indeed virtually insensitive to heating times in excess of the minimum value of about four hours over the temperature range of from about 400'to 525 C. On this figure, plotted from data resulting from experiments all carried out at 450 C. but exemplary of runs conducted at other temperatures within the described range, it is seen that while reorientation temperature may be further improved, that is, further decreased with prolonged heating, the improvement so realized is minor compared to that which results in the first four hours of treatment.

It has been indicated that the inventive processes depend upon a hydrogen-containing ambient during heat treatment. Work conducted to data has been sufficiently revealing to suggest a number of alternatives to hydrogen which will satisfy the responsible mechanistic requirement, and so result also in decreasing reorientation temperature. No such alternative, however, promises all of the advantages of the claimed process. Optical absorption studies conducted on the isostructural material gadolinium orthoferrite reveal the creation of OH groupings, assumed to correspond with reduction of Fe to 1%, under those conditions resulting in reducing reorientation temperature in the samarium compound. The number of OH groups based on absorption intensity reaches a maximum for the level corresponding with minimum reorientation temperature. Use of more drastic conditions such as significantly higher temperatures which have the effect of producing an increasing reorientation temperature appears to result also in a loss of OH groups and in a corresponding reduction of Fe to Fe It is evident that other approaches may be taken to bring about the same desired changes in oxidation state. Experimentation suggests that introduction of either of the tetravalent ions, silicon or tin, which go into iron sites or the use of other monovalent cations such as lithium may have effects similar to that of hydrogen. However, it is believed that the eventual value of the invention derives from the simplicity of the single compound, with the expectation that more perfect crystalline material will result. Use of other dopants may bring about compositional inhomogeneities such as are of concern in mixed rare earth orthoferrite compositions.

It is evident from the above description that oxidizing impurities are to be kept out of the ambient during heat treatment. It is true that rapid diifusivity of hydrogen will result in some improvement provided only that the ambient has sufficient excess of hydrogen to be reducing. Nevertheless, for optimum results, it is desired to keep such oxidizing contaminants to a minimum. To this end, a level of 50 parts per million is considered a tolerable maximum for such ambient contaminants.

Inert atmospheric components are not harmful in reasonable amount, the only requirement being that there be sufiicient hydrogen in the atmosphere to accomplish the desired result. Accordingly, the atmosphere may be produced by the direct introduction of, for example, cracked ammonia or by use of methane or any other gaseous composition which results in the introduction of hydrogen at the crystal surface. To assure sufiicient rates, it is generally preferable that the atmosphere contain at least ten per cent by volume of hydrogen.

EXPERIMENTAL PROCEDURE The general procedure followed in establishing the operating limits is described. The data plotted on FIGS.

1 1 and 2 was derived under the outlined conditions.

Crystalline sections of SmFeO; of various sizes but typically in the form of rectilinear bars about 3 mm. x 3 mm. x 5 mm. were placed in a platinum container which was inserted in a tube furnaceequipped with a sufliciently gastight tube to prevent diffusion of significant quantities of oxygen. A dry hydrogen atmosphere was maintained in the tube, the temperaturewas raised to the desired level at a rate of about 50 C./hour, the temperature was maintained for the desired period and was then dropped to a level of about 23 C. during a time interval of about four hours. In general, the hydrogen was dried by bubbling through concentrated sulphuric acid, followed by passage through finely divided asbestos fibers to prevent carryover of the acid into the tube.

Reorientation temperature was measured before and after treatment by torque measurement in a rotating magnetic field. Below the reorientation temperature, the easy direction is in the a crystallographic direction. Above the reorientation temperature the easy direction is defined along the c crystallographic direction. In the torque measuring apparatus, the crystal is so placed that the applied rotating field remains in the a-c plane. The torque required to keep the crystal from responding to the field is plotted while temperature is increased. The torque plot is read as a cyclic variation with a 90 phase change being produced through the reorientation transition.

DEVICE USES Uniaxial canted spin antiferromagnetic materials are presently of interest for use in single wall domain devices. In the operating condition, such devices generally utilize a single crystal sheet of the material so oriented that the easy direction lies normal to the sheet surface. Under these conditions, small single wall domains representing areas of magnetization direction opposite to that of the surrounding portion of the sheet may be produced. Such bubble domains may represent information, and during operation of the device may, depending on design, be obliterated, generated, or moved. It is expected that one use to which such devices will be put is as a shift register. In this use, domains are first nucleated, their presence or absence representingbinary information, and are then propagated stepwise through the sheet ultimately to a readout position in the manner of any shift register device.

The anisotropy of samarium orthoferrite or other material considered for use in such a device is large and switching magnetization direction is further complicated by the desred small effective dimension in the easy direction. Accordingly, switching is not practically accomplishable by simple application of an oppositely directed field. One method of nucleation is by the local attainment of reorientation temperature while maintaining the desired region under the influence of a field of direction opposite to that of the surrounding region. This may be accomplished by raising an entire sheet to the desired temperature and applying a local field or by use of a gross field and local heating as, for example, by use of a laser beam. The device in FIG. 3 is illustrative.

In FIG. 3, a register 20 comprises a sheet 21 of samarium orthoferrite in accordance with the invention. The sheet is so oriented that at the operating temperature the preferred magnetization direction (easy direction) is normal to the plane of the sheet. Flux directed out of the paper as viewed is represented by a plus sign. Flux directed into the paper is represented by a minus sign. Conductors 22, 23, and '24, which may be deposited on the surface of sheet 21, form triplets of loops 22a, 23a, 24a; 22b, 23b, 24b, et seq. Loop size is somewhat smaller than the size of a corresponding stable single wall domain so that in operation any magnetized domain is partly within an adjoining loop. Such domains, once nucleated, for example, by means of a domain nucleating source 25 and loop 26, are stepped from loop position 22a to 23a to 24a to 22b and so forth by successive energization of conductors 12, 13, and 14 in that order by means not shown. Readout is accomplished by means of loop '27 and sensing means 28.

Other device uses include switches, other types of memory elements, logic elements, etc. Some such devices may operate at constant temperature at or near the spin flop temperature. Others may depend on a temperature variation sometimes local to reverse the magnetization and so provide a means for easily nucleating a domain.

In other manner, the device description has been rudimentary. Devices of the type depicted in FIG. 3 have been developed to a far more advanced state. Some no longer utilize looped conductor configurations but depend upon the flux concentration which results from .a sharp turn in the conductor pattern. A simple zig-zag pattern, for example, results in a bit location at each conductor reversal position. More generally, while present interest largely centers on the use of the materials of this invention in single wall domain elements, other devices may depend upon more conventional properties such, for example, as overall changes in magnetization, in changes in transmission properties of electromagnetic energy, under the influence of an applied field or with temperature change, etc. All such device uses are to be considered within the inventive scope.

Crystalline samarium orthoferrite used in the experiments upon which much of the data is based was grown out of a lead oxide-boron oxide flux in accordance with the procedure outlined in some detail in U.S. Pat. 3,079,- 240. Generally, this procedure involves forming a complete solution of the starting ingredients samarium oxide and ferric oxide in a flux containing of the order of at least 60 percent by weight flux. Variations on this procedure are of course suitably applied to the preparation of the initial material. For example, the flux may contain only lead oxide or such additional ingredients as PbF to increase solubility or to otherwise control the crystallizing process. Growth may be by spontaneous nucleation or on a seed. Seeds may be immersed or slowly Withdrawn. Other procedures which may be used for preparation of samarium orthoferrite are known.

The invention has been described in relatively simple terms. Broadly, it is based on the discovery that hydrogen treatment at elevated temperature results in reduction of reorientation temperature in samarium orthoferrite. Reorientation temperatures attained generally center below about 100 C. and may be as low as about 85 C. Any samarium orthoferrite having a reorientation temperature substantially above this value may be so improved. Since typical untreated materials, whether grown by such diverse methods as PbO-containing flux or hydrothermally, have manifested reorientation temperatures at about 175 C., the improvement so realized is substantial.

Device uses benefiting from the lower of this temperature have been described only briefly. It is apparent that a vast array of other devices may beneficially utilize these materials, and also that certain variations may be made on the described process without departing from the underlying teaching of the invention. Similarly, it is evident that the inventive result obtains for compositions of magnetic rare earth orthoferrites other than the samarium compound and also that the same effect is obtained on mixed orthoferrites. While it is not apparent 6 that any technological advance will be realized by operating on such mixed compositions, it is nevertheless true that at least minor intentional or unintentional variations may be tolerated in the samarium orthoferrite under treatment. The appended claims are to be so construed.

What is claimed is:

1. Procedure for treating a crystalline product comprising SmFe0 characterized in that the said product is heated in an ambient which contains at least 10 percent by volume of hydrogen to a temperature of at least 400 C. and for a time sufiicient to result in a substantial lowering of the magnetic reorientation temperature to a value which centers below about C.

2. Process of claim 1 in which the said material is maintained at the said temperature for a period of at least four hours.

3. Process of claim 1 in which the maximum temperature is about 525 C.

4. Process of claim 1 in which the said ambient is substantially pure hydrogen.

References Cited Belov et al.: Chemical Abstracts, vol. 56, 1962, p. 12427.

Eibschutz et al.: Journal of Applied Physics, vol. 35, No. 3, March 1964, pp. 1071-1072.

HERBERT T. CARTER, Primary Examiner I U.S. c1. X.R. 23-41, 293 

